Conventionally, in a plasma processing apparatus for processing, e.g., a semiconductor wafer, an observation window has been used to measure particles in a processing room and/or detect an emission spectrum of plasma. For example, there has been disclosed a method for measuring particles in a plasma (see, e.g., Japanese Patent Laid-open Publication No. 1997-203704). In this method, a laser light is irradiated from a side wall of the processing room; the light is reflected or scattered when colliding against floating particles in the plasma and the particles are measured based on intensity variation of the reflected light or the scattered light observed through the observation window.
In an aspect of measuring the reflected light or the scattered light when a laser light is irradiated to floating particles in the plasma, it is preferable that the observation window for measuring the reflected light or the scattered light is set to be sufficiently large. However, in case the observation window is large, electrons which form the plasma may be introduced into the observation window. As a result, a plasma may be newly formed in the observation window to produce particles or deposits on the observation window, so that light efficiency may be deteriorated and light dispersion may occur at the location of the observation window. Therefore, a high-precision measurement of particles may be difficult to accomplish.